I believe its the nebular gas that is causing it to look flat,
One of my hobbies is astronomy and sadly it is hard to imitate nature when it comes to gas clouds in space, they almost always look flat unless someone takes the time to actually model one and texture it , or replicate it with dynamics.
Perhaps making a simple star field woudl serve this image better.

I do like the render tom, My personal preference is for the whiter settings in the shader / texture, which ever you adjusted, but the masses appear to like the darker better, so By all means continue as is..
Aside it doesn't detract from your work at all by any means, so no fear there mate.

G-man